1. Field of the Invention
This invention relates to an improved circuit breaker having a toggle link mechanism in which the separation distance between a movable contact and a stationary contact is larger for automatic or overcurrent operation than for manual operation.
2. Description of the Prior Art
Circuit breakers having a toggle mechanism are widely used in industrial, commercial, and residential environments for protecting power distribution equipment against damage from overcurrents, short-circuit currents, and other abnormal electrical conditions, as is well known. A conventional circuit breaker of this type is shown in FIG. 1, wherein reference numeral 1 designates an insulating cover, numeral 2 an insulating base which forms a housing or case for a three-phase or three-pole circuit breaker in conjunction with the cover, and numeral 3 stationary conductors which are employed in all poles, one of which is shown, each having a stationary contact 4 at its left end portion. The other ends of the stationary conductors 3 are used as connection terminals for external conductors. Numeral 5 designates a movable contact assembly provided for each pole and composed of a movable contact 6 facing the stationary contact 4 and a movable contact arm 7 fixed to a common cross bar 8 for the simultaneous opening and closing of all contacts. Numeral 9 designates an arc extinguishing chamber provided for each pole and enclosing the corresponding stationary and movable contacts 4 and 6, and numeral 10 a toggle mechanism having a first link 11 and a second link 12 and being coupled with the center one of the three poles. One of the ends of the first link 11 is pivoted to the center portion of an L-shaped bell crank 13 by shaft 14, and the other end is pivoted to one of the ends of the second link 12. The other end of the link 12 is pivoted to the center portion of the movable contact arm 7.
Numeral 16 designates a switch handle for manually opening and closing the circuit breaker or interrupter, numeral 17 an operating spring whose one end is engaged with the handle 16 and the other end with the shaft 15 of the toggle link mechanism 10. The spring 17 is expanded and under tension in the closed position of the movable contact 6. Numerals 18 and 19 designate a thermal tripping mechanism and a magnetic tripping mechanism, respectively, both of which are designed to rotate a trip bar 22 around a pivot pin 22A in a counterclockwise direction by their action. The trip bar 22 is rotated by the thermally induced bending action of a bimetallic strip 20 of the thermal trip mechanism 18 when the current flowing through the strip increases to an overcurrent level, or by the magnetic attraction of a movable trip yoke 21 when the instantaneous current rises to an extremely high level, that is, a short-circuit current. Numeral 23 designates a spring biased latch whose one end 23A is engaged with one portion of the trip bar 22, i.e., one end 21A of the movable trip yoke 21, and whose other end 23B is held engaged with one end 13A of the crank 13 during both the closed and manually open conditions of the movable contact 6.
If the handle 16 is pulled towards the closed position when the crank 13 is engaged with the latch 23, the shaft 15 is moved to the right, and the toggle link mechanism 10 extends or straightens. The further movement of the shaft 15 is limited when the toggle link mechanism strikes the shaft 24 of the crank 13, by which time the movable contact 6 has engaged the stationary contact 4 in accordance with the extending action of the toggle link mechanism.
When the handle 16 is pulled to the open position, i.e. to the left in FIG. 1, the toggle link mechanism 10 is pivoted about the shaft 15. This pulls up on the movable contact arm 7 and rotates it in a counterclockwise direction around and with the cross bar 8 to open or break the contacts 4, 6. Such rotation is limited by the arm 7 striking against the crank pivot shaft 24, commonly called the first stop.
If there is an overcurrent or a transient peak in the circuit the contact between the crank 13 and the latch 23 is broken, the released crank rotates in the clockwise direction around the shaft 24, and is limited in its rotation by striking against the stationary shaft 25, commonly called the second stop. In this automatically opening action, as the shaft 14 connecting the crank 13 with the upper link 11 crosses over the line of action of the spring 17 due to the release of the crank by the latch 23, the toggle link mechanism 10 is pivoted about the shaft 15 by the restoring force of the extended spring. The flexing of the toggle link mechanism causes the counterclockwise rotation of the movable contact arm 7 and cross bar 8, whereby the contacts 4, 6 of each pole are opened simultaneously.
To improve this conventional circuit breaker it is desirable that the first stop, i.e., the shaft 24, to disposed at a higher position within the housing in order to increase the current interrupting capacity by lengthening the separation or arcing distance between the opened movable and stationary contacts 6, 4. In other words, a higher position of the first stop 24 would permit the rotation of the contact arm 7 over a larger angle, i.e., a larger moving stroke of the contact 6. It would also result in a smaller angle .theta. between the flexed upper and lower links 11, 12 when the contacts are open, however, and this would make it considerably more difficult to reclose the contacts. To overcome such difficulty a larger throw angle of the handle 16 would be required so that the center line of the spring 17 could pass over the shaft 14 during the clockwise rotation of the handle around the shaft 15 to the closed position, and a larger force from the spring 17 would also be required.
Such compensating modifications would be difficult, however, due to the size limitations imposed by the circuit breaker case, the size of the altered parts, and/or their cost, and the present invention thus seeks to overcome these difficulties.